Hello Jginley712
You cannot change the tickets after purchase. It would require a full cancellation and a repurchase. Whether this is possible? It depends on the ticket fare, you have to check with SBB.
International tickets can only be changed by calling SBB or by seeing a travel agent in the SBB travel centre in Switzerland. Phone: +41 51 225 78 56 (24/7, waiting times may occur).
It might be that the entire hassle of changing tickets will cost you more than you actually gain by changing the ticket. The decision is yours.
Kind regards,
Roland